1. Field of the Invention
The present invention relates to a reflective display device including a retroreflector and a method for evaluating the retroreflector.
2. Description of the Related Art
Various configurations have been proposed for a reflective liquid crystal display device with a retroreflector (see Japanese Laid-Open Publication No. 2002-107519, Japanese Patent No. 3216584, and Japanese Laid-Open Publication No. 2002-287134, for example). Each of those reflective liquid crystal display devices uses no polarizers and therefore can conduct a display operation with increased brightness, and is also expected to achieve a display at a higher contrast ratio. As used herein, the “retroreflector” refers to an optical element that has a two-dimensional arrangement of very small unit elements so as to reflect any incoming light ray back to the source by way of multiple reflective surfaces thereof, no matter where the light ray has come from.
FIG. 19 shows an exemplary configuration for a reflective display device with a retroreflector as disclosed in Japanese Laid-Open Publication No. 2002-107519.
The reflective display device 9 shown in FIG. 19 includes an electrode 4, an alignment film 2, a liquid crystal layer 1, another alignment film 3, another electrode 5 and a retroreflector 8, which are stacked in this order (as viewed from over the display device by a viewer) between two substrates 6 and 7. The liquid crystal layer 1 is made of a scattering type liquid crystal material, which can switch between a transmitting state of transmitting the incoming light and a scattering state of scattering the incoming light.
Hereinafter, it will be described how the reflective display device 9 conducts a display operation in principle.
First, when the liquid crystal layer 1 is in the transmitting state, an incoming light ray 10, which has come from the vicinity of viewer's eyes, is transmitted through the substrate 6 and liquid crystal layer 1 while being refracted at the same time, incident on the retroreflector 8 and then reflected back from the retroreflector 8 as a reflected light ray 11. The reflected light ray 11 returns to the vicinity of the viewer's eyes after having been subjected to a similar refraction. Meanwhile, any other incoming light ray, which has come from elsewhere than the vicinity of the viewer's eyes, is retro-reflected by the retroreflector 8 right back to its source and never reaches the vicinity of the viewer's eyes. As a result, only the incoming light ray 10 that has come from the vicinity of the viewer's eyes is sensed by him or her, thereby achieving a black display state. Next, when the liquid crystal layer 1 is in the scattering state, light entering the liquid crystal layer 1 is either backscattered or forward scattered by, or transmitted through, the liquid crystal layer 1. The backscattered light returns to the viewer, thus contributing to the white display mode. On the other hand, the light rays that have been forward scattered by, or transmitted through, the liquid crystal layer 1 are retro-reflected by the retroreflector 8 and then enter the liquid crystal layer 1 in the scattering state again so as to be subject to the scattering action of the liquid crystal layer 1. Accordingly, most of the light that has been retro-reflected by the retroreflector 8 returns to the viewer and used to achieve the white display mode. In this manner, not only the light that has been backscattered by the liquid crystal layer 1 but also the light that has been either transmitted through, or forward scattered by, the liquid crystal layer 1 can be used for display purposes as well. Consequently, a display with a high brightness is achieved.
To operate the reflective display device 9 according to this principle, the arrangement pitch of the unit elements of the retroreflector 8 needs to be at most approximately equal to, and preferably smaller than, the pixel pitch. If the arrangement pitch of the unit elements is greater than the pixel pitch, then the incoming light ray 10, which has been transmitted through a pixel of the liquid crystal layer 1 and then retro-reflected by the retroreflector 8, may pass through another pixel of the liquid crystal layer 1 on the way back. In that case, the display might exhibit an abnormal state. For example, an incoming light ray that has passed a red color filter before reaching the retroreflector 8 may pass through a green or blue color filter on its way back, thus possibly causing a color mixture unintentionally.
The display performance of the reflective display device 9 heavily depends on the retroreflection property of the retroreflector 8. Among other things, the brightness of the black display mode is substantially determined by the retroreflectivity of the retroreflector 8 in many cases. That is to say, the higher the retroreflection property of the retroreflector 8, the greater the ratio of the brightness (or luminance) of the white display mode to that of the black display mode (i.e., the contrast ratio) and the higher the quality of the display realized.
Accordingly, for a reflective display device with a retroreflector such as the reflective display device 9 to achieve excellent display performance, the retroreflector 8 thereof needs to be a reflector that includes the unit elements at a sufficiently small arrangement pitch and has high retroreflection property.
Examples of reflectors functioning as the retroreflector 8 include a reflector obtained by densely packing spherical beads and a reflector obtained by regularly arranging unit elements such as corner cubes. Among these various types of reflectors, a reflector with an arrangement of corner cubes (which is often called a “corner cube reflector”) is generally believed to achieve the highest possible retroreflection property. In a reflector densely packed with beads on the other hand, a gap is created inevitably between the beads, no matter how densely those beads are packed, and such a gap never contributes to retroreflection. For example, in a reflector, which is two-dimensionally packed most densely with beads of the same diameter, the percentage of the total area of those non-retroreflective portions (i.e., the gaps) to the overall surface area is estimated to be as high as slightly less than 10% (e.g., 9.3%) per unit area. Meanwhile, in a reflector with an arrangement of triangular pyramidal concave portions among various retroreflectors called “corner cube reflectors”, the percentage of the total area of non-retroreflective portions to the overall surface area is estimated to be about 30% per unit area. As can be seen, in those reflectors obtained by densely packing beads or arranging triangular pyramidal concave portions, the percentage of the non-retroreflective portions to the overall reflector is too high to achieve sufficiently high retroreflectivity. On the other hand, in a square corner cube reflector (i.e., a reflector with a square corner cube array obtained by regularly arranging a plurality of unit elements, called “square cube corners”, each consisting of three square planes that are opposed perpendicularly to each other) among various corner cube reflectors, the percentage of those non-retroreflective portions is estimated to be zero in a plan view, theoretically speaking. Thus, such a square corner cube reflector is expected to achieve sufficiently high retroreflection property. As used herein, the “corner cube” or “square cube corner” includes a structure having a substantially corner cube shape or a substantially square corner cube shape. More specifically, a square cube corner is a structure having at least three sets of mountain lines and valley lines.
In view of these considerations, if a square corner cube reflector is used as the retroreflector 8, then a high retroreflection property should be achieved theoretically speaking and a high-quality display could be realized.
Actually, however, it is extremely difficult even to make a square corner cube reflector at such a small arrangement pitch (of 250 μm or less, for example). None of the cited references mentioned above (namely, Japanese Laid-Open Publication No. 2002-107519, Japanese Patent No. 3216584, and Japanese Laid-Open Publication No. 2002-287134) provides a specific method of making a square corner cube reflector at that small arrangement pitch. Likewise, none of the other conventional methods of making square corner cubes mechanically such as a plate method and a pin bundling method is suitable to making a square corner cube reflector at that small arrangement pitch.
Meanwhile, Japanese Laid-Open Publication No. 7-205322 discloses a method of making a square corner cube array by a photochemical technique. In this method, a photoresist film is patterned with a mask having a plurality of equilateral triangular transparent regions. Each of these transparent regions of this mask has variable transmittance that gradually decreases from its center toward its periphery. By performing exposing and developing process steps with such a mask, a number of triangular pyramidal photoresist pattern elements are formed on a substrate. Then, the substrate, which is partially covered with those photoresist pattern elements, is etched by a predetermined technique so as to have a plurality of protrusions in the same shape as the photoresist pattern elements. In this manner, an array of corner cubes can be formed on the substrate.
Furthermore, a technique of forming a cubic corner cube of a very small size, consisting of three square planes that are opposed perpendicularly to each other, is described in “Precision Crystal Corner Cube Arrays for Optical Gratings Formed by (100) Silicon Planes With Selective Epitaxial Growth”, Applied Optics Vol. 35, No. 19, pp. 3466-3470. According to this technique, an oxide film for use to control the crystal growth is locally provided on (111) planes of a silicon substrate to cause an epitaxial growth of crystals on the substrate, thereby forming an array of corner cubes of a very small size thereon.
Thus, according to the non-mechanical method disclosed in Japanese Laid-Open Publication No. 7-205322 or Applied Optics Vol. 35, No. 19, pp. 3466-3470, a square corner cube reflector can be formed at an even smaller arrangement pitch but it is still difficult to make a square corner cube reflector with a sufficiently high retroreflection property.
The reason is that the retroreflection property of a square corner cube reflector depends heavily on the shape precision of each of the three square planes that make up one unit element (i.e., a single square corner cube), the planarity of each of those planes (i.e., the angular precision of each plane) or the precision of a joint portion between two adjacent planes, all of which will be referred to herein as “shape precision” collectively. According to the non-mechanical methods mentioned above, it is difficult to make a square corner cube array in an almost ideal shape, and therefore, the actual retroreflection property deteriorates significantly from its theoretical one.
More particularly, as for a square corner cube obtained by a photochemical method as disclosed in Japanese Laid-Open Publication No. 7-205322, it is difficult to ensure high plane precision (i.e., planarity). In that method, the plane precision of each side surface of a square corner cube depends on that of a triangular pyramidal photoresist pattern element on the substrate. However, to increase the plane precision of the photoresist pattern element, the processing steps of exposing and developing the photoresist layer should be controlled strictly enough by making the variation in transmittance or opacity of the mask constant, for example. Actually, though, such a strict process control is hard to realize.
Furthermore, according to the method utilizing the selective growth of silicon as disclosed in Applied Optics Vol. 35, No. 19, pp. 3466-3470, it is difficult to control the lateral growth of crystals. Also, a silicon dioxide film to be deposited on a silicon substrate to determine the square corner cube pattern and a film to be stacked thereon are likely deformed significantly at the end surfaces thereof. Thus, it is not easy to make a square corner cube array in its intended shape by such a method, either.
Thus, it has been very difficult to make a square corner cube array, consisting of unit elements that are arranged at a sufficiently small pitch (e.g., 250 μm or less), with high accuracy. In particular, the smaller the arrangement pitch of unit elements, the lower the shape precision of the square corner cube array and the less satisfactory the resultant retroreflection property tends to be. Accordingly, when the reflective display device is fabricated using a retroreflector 8 including the conventional square corner cube array, the quality of black display mode deteriorates particularly significantly. As a result, the desired display performance (such as a contrast ratio) cannot be achieved.
Meanwhile, Japanese Laid-Open Publication No. 2002-107519 discloses a configuration for improving the quality of black display mode, and eventually the display performance, by providing a light absorbing surface portion for each unit element of the retroreflector 8. In such a configuration, the contrast ratio can be increased significantly with the retroreflectivity of the retroreflector kept low (e.g., 40% or less). Consequently, high-quality display is realized.
When the configuration of Japanese Laid-Open Publication No. 2002-107519 is adopted, however, the light absorbing surface portion needs to be provided for the unit element of a very small size, thus complicating the manufacturing process too much. What is worse, the retroreflection property of the retroreflector slightly deteriorates, and therefore, the quality of white display mode cannot be improved. For that reason, this configuration can increase the contrast ratio to only a limited degree.